The present invention relates in general to planar techniques for the manufacture of semiconductor devices and in particular to a method for the manufacture of substrateless thin-film field-effect devices such as, for example, organic thin-film transistors.
Organic-based transistors have been the subject of investigation for less than twenty years and their development is currently having a decisive boost. These devices are basically field-effect devices which are based on a design known as thin-film transistor (TFT), which was initially devised for devices made of amorphous silicon.
Up to now, devices having one of the two structures shown in FIGS. 1a and 1b and known as the “top-contact” type and the “bottom-contact” type, respectively, have been produced with vertical geometry on the basis of this design.
In the first example of the prior art illustrated, the structure is formed on the basis of a rigid substrate SUB on which the gate contact G, the insulating layer I, the organic semiconductor layer SC and, finally, the source and drain contacts S and D are formed in succession.
In the second example, the structure is again formed on the basis of a rigid substrate SUB and the gate contact G, the insulating layer I, the source and drain contacts S and D and, lastly, the organic semiconductor layer SC are formed thereon in succession.
The device is defined as “organic” because the semiconductor in which the conduction channel is formed, in which channel the flow of current takes place between the source and drain, is made of an organic material (generally a polymer) rather than of silicon or other inorganic compounds. This structure can—in principle—be produced on flexible substrates (for example, on plastics) and this gives access to a wide range of applications which require flexibility and are incompatible with technology based on silicon or on other semiconductors having characteristics of mechanical rigidity.
In contrast with silicon-based technology, the development of which is directed towards satisfying requirements for a very high degree of miniaturization and high speed, the technology of the development of organic semiconductors satisfies the opposite requirements of applications on large areas and/or for which a slow switching speed is adequate.
A structural design of a device with horizontal geometry is shown in FIG. 1c. The substrate SUB is made of highly doped silicon and the gate contact G and the insulating layer I, generally silicon oxide, are formed side by side thereon so as to produce the gate in an outer region relative to the channel and to be able to contact it easily. The source and drain contacts S and D are formed on the insulating layer. The organic semiconductor SC is deposited between the source and drain contacts. The highly doped substrate behaves as a metallization to form the gate contact and the insulating oxide layer permits capacitive coupling of the gate contact with the channel region which is formed in the semiconductor.
As an alternative to these types of rigid structure, flexible plastics substrates, on which the successive layers are deposited, are used. The insulating layer is generally deposited from the liquid phase by a spin-coating or auto-assembly technique (as is the semiconductor), whereas the source, drain and gate contacts are generally formed by thermal evaporation of metals or, according to the most recent techniques, by screen-printing or soft lithography with materials of an organic nature and in the liquid phase.
Applications of flexible organic thin-film transistors relate, for example, to the development of so-called “electronic paper”, which is a product that consists of a biplanar structure in which a special electric-field sensitive ink, for example, containing electrically charged microspheres, is inserted between two sheets. The application of a predetermined voltage between the two sheets (each of which has a matrix of electrodes, and at least one of which is transparent), or an absence of voltage, leads to the appearance of the ink (dark) or of the background (light) on the transparent “reading” side. For example, in the form with ink microspheres, the variation of the voltage applied between the matrices of electrodes causes the microspheres to migrate from one side of the sheet to the other.
The entire surface is arranged in pixels which can be addressed separately and text or graphic compositions preselected by the user appear, according to the voltages applied. Whatever technique is used with regard to the ink, each pixel requires an electronic driver circuitry which must also be formed on a flexible substrate.
A field-effect transistor which is disposed on a substrate with properties of flexibility and is manufactured on the basis of an insulating polyester film with polymer materials by printing techniques is described in the article “All-Polymer Field-Effect Transistor Realized by Printing Techniques” by F. Garnier, R. Hajlaoui, A Yassar and P. Srivastava which appeared in the journal “Science”, Vol. 265 of 16 Sep. 1994.
U.S. Pat. No. 6,326,640 describes an organic thin-film transistor according to various configurations in which greater mobility of the charge carriers is achieved by the provision of a layer of alignment of the molecules of the organic semiconductor between the source and drain contacts. However, no techniques other than those normally adopted are indicated for the manufacture of the device in the top-contact and bottom-contact configurations.
Various disadvantages can be pointed out with regard to the known forms mentioned above.
The structure of FIG. 1c has the disadvantage that, by depositing the organic semiconductor in a non-targeted manner (for example, by spin-coating) it is deposited on the entire free surface, well beyond the channel region in which it is required, creating undesired conductive paths between source and gate or between drain and gate. To prevent this problem, in known manufacturing processes, a further technological step is performed and consists in masking of the channel region and/or in subsequent cleaning of excess organic semiconductor.
The structures of FIGS. 1a and 1b intrinsically solve the problem of the deposition of the insulation on a circumscribed area of the substrate, leaving the gate region exposed, but present problems in the alignment of the masks, which constitute an obstacle to the miniaturization of the devices.